Capacitive power transfer (CPT) and inductive power transfer (IPT) are two effective methods to transfer power wirelessly. The CPT technology utilizes high-frequency alternating electric fields to transfer power without direct electric connection, while the IPT system uses magnetic field to transfer power. The IPT technology has already been widely used in many applications, such as portable electronic devices, biomedical devices, and electric vehicle charging.
Compared with the IPT system, the CPT system has many advantages. Magnetic fields are sensitive to nearby metal objects and the system efficiency drops quickly with this interference. They also generate eddy current losses and, hence, heat in a conductive object, which creates a potential fire hazard. However, the electric field in the CPT system does not generate significant losses in the metal objects.
The recent CPT system can be classified by the matching network topology. The most popular topology is a single inductor resonating with the capacitor to form a simple series-resonant circuit. The second topology is the LCL structure at the front-end to step-up the voltage for the coupling capacitor. However, there is also an inductor directly connected with the capacitor to form a series resonance. In these two topologies, the series inductance is large because of the small value of capacitance. The voltage pressure on the capacitor is also large. The third topology is the resonant class E converter or the nonresonant PWM converter, used to replace the compensation inductor. All of these systems require very high capacitance values, in the tens or hundreds of nanofarad range. So, the transferred distance is usually around 1 mm.
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